In the past there have been two primary forms of bore gages, specifically a mechanically operated bore gage in which a moving contact point is mounted in a head with some centralizing device. The motion of the movable contact is transmitted to a dial indicator or other indicating means by a mechanical linkage of some sort as, for example, shown in U.S. Pat. No. 3,418,720. There are also provided in the art an electronic version of this arrangement where contact points through the medium of a leverage system transmit their motion to devices such as linear voltage differential transformers (LVDT) as shown by U.S. Pat. No. 2,642,671. A still further form of bore gage is a probe having a diameter close to the diameter of the bore to be measured; that is, slightly undersize on the order of say 5 thousandths of an inch. In directing air jets from this measuring head the principal operation of this latter form is shown in U.S. Pat. No. 2,846,871. In many cases bore gages must operate in rather dirty environments with coolant, chips, and other foreign matter being present. For various reasons the prior art schemes have not proven to be entirely satisfactory since there are a number of places where dirt can enter the system and upset any measurement that is being achieved.